Culture of any company is defined by its leadership. Allowing an employee to speak disrespectfully or act in a way that demoralizes other contributors has to be dealt with quickly and specifically.

This person must be given specific examples of behaviour that leadership is saying is not acceptable and that any further actions like those examples will result in their immediate termination.

It doesn't matter that this person is a top contributor. You are doing far more harm to the long-term value of your company by allowing such behaviour to be tolerated.

Good leaders never live in fear of their employees (e.g what if this person leaves?). If anything employees should fear failing to live up to the expectations set by leadership.

A company should have one set of non negotiable rules that *everyone* must adhere to or face the *same* consequences.

There should be a lot of flexibility in any company especially a startup bit your non-negotiables must be firm.

This person might be the top-trained but isn't worth keeping around if they can't adhere to the rules.

There is a bigger issue here which is the way that you or your CEO is running the company. Put simply, you can't live in fear of your technical talent. Your company will always be limited in its success in this scenario. I can help you define the language, rules and cultural DNA you or your leadership requires to be effective, recruit great talent and have a cohesive, enjoyable work environment.